Arsenal suffered a major setback in their Barclays Premier League title challenge as Gabriel Agbonlahor earned victory for Aston Villa at the Emirates Stadium.
Ashley Young had a first-half penalty saved by Manuel Almunia but then helped set up the opener when he crossed for Agbonlahor, who celebrated despite Gael Clichy appearing to get the final touch.
Agbonlahor then added the second 10 minutes from time when Villa broke, with Arsenal cancelling out the good work of last week when they beat Manchester United.
Villa were outclassed by Chelsea during their last trip to London but there was no repeat and they should have gone into the interval ahead, regardless of Young's miss from the spot.
Arsenal's passing was sloppy and their attack lacked an edge while they had Nicklas Bendtner up front on his own, with Emmanuel Adebayor only on the bench after recovering from an ankle injury.
Wenger used his programme notes to highlight the speed of Agbonlahor and Young and they caused the hosts all sorts of problems, with a warning sign coming in the opening minute when Young fired straight at Almunia.
dictating play
Brad Friedel fumbled a straightforward shot from Cesc Fabregas, with Curtis Davies required to hack the ball clear as William Gallas lurked, but apart from that it was Villa dictating play.
Young missed his penalty in the 20th minute after earning the spot-kick himself.
Mikael Silvestre had cleared only as far as Young after a deep cross caused chaos, with Theo Walcott sliding in when the Villa winger surged back in the box.
Almunia went the right way with the weak penalty but it still required a brave tackle from Gallas as Young chased the rebound.
Young may not have even been taking the penalty had Gareth Barry not been injured in the build-up, with Fabregas getting a yellow card for clattering into the England midfielder.
Mazy run
Villa were still taking the match to the hosts and former Arsenal trainee Steve Sidwell volleyed straight at Almunia.
Young almost atoned for his penalty miss with a drive that went just wide of Almunia's post, and he had another effort that went straight at the Arsenal goalkeeper after a mazy run.
Almunia was made to work harder when Agbonlahor pulled the ball back for Barry, the Spaniard down sharply as Villa threatened again.
Arsenal were struggling for fluency so Wenger made his move just after the hour mark, with Adebayor coming on to replace Abou Diaby.
Now playing a 4-4-2 formation, Arsenal appeared more threatening, although Wenger still looked frustrated on the touchline. Vela was the next to come on, replacing Bendtner, but it was Villa who took the lead.
Sagna was injured during the build up after falling awkwardly, with Villa raiding forward. Young crossed for Agbonlahor and the ball glanced past Almunia into his far corner as Clichy challenged.
Agbonlahor claimed it and there was no argument about the second when he chased a long ball, held off Gallas and fired home.
